As illustrated in FIG. 8, when an electromagnet 27 of an electromagnetic clutch mechanism 24 is electrically excited, an armature 25 is electrically attracted by the electromagnet 27 such that a pilot frictional clutch 26 is brought into a frictionally engaged state. When a relative rotation is then produced between an outer casing 14 and an inner casing 17, a torque T is transmitted to a pilot cam member 32. For example, when the vehicle is driven in a forward direction during the frictional engagement of the pilot frictional clutch 26, the pilot cam member 32 is transmitted with the torque T in an arrow direction illustrated in FIG. 9(a). On the other hand, when the vehicle is driven in a rearward direction or when engine braling is performed, the pilot cam member 32 is transmitted with the torque T in an arrow direction illustrated in FIG. 9(b) which is opposite to the direction illustrated in FIG. 9(a). In whichever driven condition the vehicle is, the torque T is amplified by a cam mechanism 23 including a main cam member 31, a spherical cam member 38, and the pilot cam member 32 and is transformed to a thrust S along rotational axes 14a and 17a. A main frictional clutch 22 is brought to a frictionally engaged state depending on the thrust S, thereby enabling to transmit the driving force between the outer casing 14 and the inner casing 17.
When the electric excitation to the electromagnet 27 is terminated, the armature 25 is released from being attracted by the electromagnet 27. In this case, the pilot cam member 32 and the main cam member 31 integrally rotate via the spherical cam member 38 as illustrated in FIG. 9(c). The main frictional clutch 22 is then released from the frictionally engaged state, thereby interrupting transmitting the driving force between the outer casing 14 and the inner casing 17.
However, in the above-described condition where the electromagnet 27 has not been electrically excited, inner plates of the pilot frictional clutch 26 may be dragged by outer plates thereof due to high viscosity of oil in the driving force transmitting apparatus 1 at a relatively low temperature. This unfavorable dragging may occur especially when a rotational speed of the vehicle front wheel is lower than one of the vehicle rear wheel in the above-described condition. In this case, the pilot cam member 32 may be transmitted with a drag torque t in an arrow direction illustrated in FIG. 9(c). The drag torque t is amplified by the cam mechanism 23 in the same manner as the torque T illustrated in FIG. 9(b). The amplified drag torque t is transformed to the thrust S and is transmitted to the main frictional clutch 22. Therefore, the main frictional clutch 22 may be brought to the frictionally engaged state, thereby unnecessarily transmitting the driving force between the outer casing 14 and the inner casing 17. This may cause instable control of the driving force transmitting apparatus 1.
In light of foregoing, in the conventional driving force transmitting apparatus 1, a return spring 51 such as a disc spring is provided for biasing the main cam body 31 along the rotational axes 14a and 17a in a direction against the thrust S. The return spring 51 preferably acts for preventing the main frictional clutch 22 from being brought to the frictionally engaged condition. In the meantime, at least following three problems may possibly occur by providing the return spring 51 in the conventional driving force transmitting apparatus 1.                1) In order to reduce rolling resistance of a needle roller bearing 35 supporting the pilot cam member 32, an urging force of the return spring 51 is required to be reduced as well. Meanwhile, in order to restrain the drag torque t which occurs due to the high viscosity of the oil in the driving force transmitting apparatus 1 at the relatively low temperature, the urging force of the return spring 51 is required to be increased. Therefore, according to the conventional driving force transmitting apparatus 1, the drag torque t may not be effectively restrained concurrently with reduction of the rolling resistance of the needle roller bearing 35.        2) The drag torque t is amplified and transformed to the thrust S of the main cam member 31, and the return spring 51 then counteracts the thrust S of the maim cam member 31. The return spring 51 possesses the urging force for biasing the main cam member 31 in the direction against the thrust S along the rotational axes 14a and 17a. At this point, the pilot cam member 32 is applied with the urging force of the return spring 51 via the spherical cam member 38, which urging force is set to be relatively large in its degree to counteract the thrust S. Therefore, the rolling resistance (i.e. a frictional resistance) of the needle roller bearing 35 supporting the pilot cam member 32 may be increased.        3) Not only under the above-described electrically nonexcited condition of the electromagnet 27 as illustrated in FIG. 9(c), the return spring 51 biases the main cam member 31 in the direction against the thrust S along the rotational axes 14a and 17a also under the electrically excited condition thereof as illustrated in FIGS. 9(a) and (b). Therefore, when the electromagnet 27 has been electrically excited, the control of the conventional driving force transmitting apparatus 1 may easily get instable due to uniformity of the urging force of the return spring 51.        
A need thus exists for providing a driving force transmitting apparatus capable of overcoming the above-described possible problems and restraining the drag torque concurrently with reduction of the rolling resistance (i.e. the frictional resistance) of the pilot cam member and the main cam member.